Currently, the data storage methods that are mainly distributed are specially suited to media such as CDs, DVDs or Blu-ray® disks. The method of writing data on such media can be carried out on a few superposed layers of the disk in 2D.
The Blu-ray medium enables the largest 2D data-storage volume, and it is generally close to or slightly greater than 50 Gbits. This capacity is in any case theoretically limited to 100 Gbits.
The generally accepted lifetime of such a medium is 10 years. This limitation is particularly due to the short lifetime of the polymer layer.
Today, for some requirements for storage, particularly for permanent archiving of data, the lifetime and the capacity of the media are too small. This limitation results in an increase in the number of media to meet the archiving and storage requirements.
New 3D data recording techniques have been developed, particularly using multiphoton absorption of a material subjected to the high intensity of a laser beam.
Two 3D recording writing methods are known—a first method of recording by so-called “photochromism” and a second method of recording by so-called “fluorescence” on glass media.
Photochromism recording particularly causes a reversible transformation of the dopant incorporated in a transparent medium. The dopant includes two states depending on the irradiation to which it is subjected, each of the states corresponding to a modification of the absorption coefficient and of the refractive index of the medium. These two states enable the light to be multiplexed differently according to the polarization, wavelength and phase features. These features can be selected such as to use the absorption and refraction properties modifying the waves reflected during lighting for the reading of data and can therefore be used to decode the information read.
A problem of photochromism recording is that reading the information, for example, by using polarization is extremely difficult to control. Indeed, the light for lighting the medium is optionally modified according to the refractive index of the lit area of the medium, but it is extremely difficult to control the polarization of the light on consumer optical components, which results in using error correctors. Furthermore, this technique requires the use of a writing-tuneable laser. Consequently, it is difficult to implement and it is expensive. Finally, the gain in the storage of data is not large enough in view of the implementation of such a solution.
Fluorescence recording has never been implemented since it has never been possible to stabilize the fluorescent species in a transparent medium. This limitation is due to the modification of the intensity of fluorescence over time particularly resulting from processes of reading the medium which consist of successive lighting. Fluorescence recording could not then allow stability and permanence of the encoded information.
Consequently, this type of technology did not enable until now an area of a transparent medium to be irradiated with a view to encoding information at the level of the fluorescence of the photo-induced species.
Furthermore, irradiation generally brings about a modification of the refractive index which does not enable reading of the data in 3D due to the diffusion of the light of the irradiated areas.